Coffee is prepared from the roasted ground beans of the plants of the genus Coffea, generally from the species C. arabica. Coffee plants produce the alkaloid caffeine, which is present in their dried fruit, coffee beans. Because many coffee drinkers prefer coffee without caffeine, a number of processes have been developed to remove caffeine from coffee beans. All of these processes result in the removal of substances other than caffeine from the beans, thereby adversely affecting the taste of coffee brewed from the treated beans. Although a few naturally occurring caffeine free coffees and related genera are known (Mascarocoffea spp. and Coffea bengalensis), they have no commercial value. (Charrier and Berthaud, "Variation of Caffeine Content In The Coffea Genus", Cafe' Cacao The', 14:251-264 (1975)). Accordingly, there is a need for a method for producing decaffeinated coffee beans that does not result in the removal of substances from the beans other than caffeine.
Caffeine is a naturally occurring purine alkaloid produced by coffee and tea plants, among others. It is believed that caffeine synthesis protects the plants from insects. Coffee plants synthesize caffeine from the nucleoside xanthosine in four sequential reactions as shown in FIG. 1. For review see Suzuki, T., Ashihara, H. and Waller, G. R., Phytochemistry 31:2575 (1992). The first step in the pathway is the methylation of the nucleoside xanthosine by S-adenosylmethionine, which is catalyzed by the enzyme xanthosine N.sup.7 methyl transferase (XMT). The product, 7-methylxanthosine is hydrolyzed (a ribose is removed) to 7 methylxanthine, and undergoes further methylations to theobromine and caffeine. It is to be expected that interruption of this sequence of synthetic reactions would block caffeine synthesis.
Accordingly, a strategy for selectively eliminating caffeine from coffee plants is to prevent synthesis of specific enzymes in the pathway for caffeine biosynthesis. In one embodiment this invention relates to genetic alteration of coffee plants to eliminate synthesis of XMT. In the presently preferred embodiment, synthesis of XMT is suppressed by transforming coffee plants with a DNA sequence that codes on transcription for a messenger RNA (mRNA) that is antisense to the mRNA that codes on expression for XMT. The invention may be generalized to produce other caffeine free beverages and food products, including tea, cocoa, and other chocolate-based beverages or foods.